Shadowing, as I understand it (and as I did it) is when you go to a dentist's office and follow them around all day to see what kinds of things they do, e.g. procedures on patients.
Volunteering can range, I suppose. My volunteering hours were geared towards a dental clinic, where I would rotate between different departments; did paperwork, cleaning of instruments, interacting with patients and the student dentists.
As far as I can gather, it's much more important to "shadow" than to "volunteer."
aranjuez